Tunable laser wavelength modulation spectroscopy is finding widespread use in various applications. One such application is the quantification of the amount of chemical species (the measurand) in a substance and in particular in an artificial or natural process such as an industrial, medical or physiological process gas analysis where traditional techniques are either unsuitable or poor.
A typical system consists of a tunable laser source such as a tunable diode laser (TDL) that emits a beam of light that is focussed on a detector. The substance that is to be analysed is positioned between the tunable laser source and the detector, so that the light incident on the detector has been modified by its passage through the substance. The modifications to the light enable various parameters of the measurand to be determined by a signal processing system that is coupled to the detector. In some cases the substance to be analysed is a gas produced by an industrial process, and the measurand may be one or more chemical species that are present in this process gas. Examples of measurand species include but are not limited to gaseous water, O2, CO and CO2 and hydrocarbons such as methane. The presence and/or amount fraction (concentration) of one or more of these measurand species may be determined using one or more TDLs.
In operation of the laser gas analyser system, the wavelength of the beam emitted by the TDL is scanned over one or more absorption lines of the measurand. At certain specific wavelengths within the range of wavelengths scanned, light is absorbed by the measurand and these spectral absorption lines can be detected by measuring the light transmitted through the substance to be analysed. This allows the necessary spectroscopic information to be acquired to determine not only the amount fraction of the measurand, but also the influence of pressure, temperature or background mixture composition. In some cases it is possible to use a single laser source to measure a plurality of measurands. In these cases, the output wavelength of the laser source is swept across a wavelength range that includes at least one discernable absorption line for each of the plurality of measurands.
In a well-designed system, wavelength modulation techniques offer very high sensitivity and enhanced spectral resolution. In particular, second harmonic wavelength modulation spectroscopy is well suited to gas analysis due to its ability to cope with a wide variety of spectroscopic situations found in industrial processes that includes congested absorption spectra, sensitive trace level measurements and obscured optical transmission.
The detector response is proportional to the intensity of light transmitted through the substance under test, which is dependent on the intensity of light transmitted through the substance under test as well as the amount fraction of measurand at the absorbing wavelengths. Therefore fluctuations of the light intensity caused by either the laser source or transmission through the analysed substance will cause uncertainty in the measurement of the amount fraction of measurand.
This is shown by the following relationships, where equation [1] represents the Beer-Lambert law of optical absorption, wherein u is the molecular density per unit length of the measurand, I is the detected amount of light, I0 is the incident amount of light (equal to unabsorbed amount when the molecular density is zero), v is the frequency of light and k is the absorption coefficient.
                              log          ⁡                      [                                          I                ⁡                                  (                  v                  )                                                                              I                  0                                ⁡                                  (                  v                  )                                                      ]                          =                  -                      uk            ⁡                          (              v              )                                                          [        1        ]            
The change in the amount of light detected at any particular frequency (∂I(v) is related to the molecular density change (∂u) as given by equation [2], which also shows the uncertainty term due to variation of the incident amount of light (∂I0)
                              ∂          u                =                                                            ∂                                                      I                    0                                    ⁡                                      (                    v                    )                                                                                                I                  0                                ⁡                                  (                  v                  )                                                      -                                          ∂                                  I                  ⁡                                      (                    v                    )                                                                              I                ⁡                                  (                  v                  )                                                                          k            ⁡                          (              v              )                                                          [        2        ]            
Variations in incident light intensity at the detector may be caused by a number of factors other than the concentration of the measurand. For example, variations can be caused by intrinsic fluctuations in the laser output, changes in ambient light intensity levels and/or obscuration in the process sample stream, which may be caused by any combination of dust, tar, corrosion or optical beam misalignment. Obscuration and changing of the intensity of ambient light are to be expected in a furnace. If the variation in incident light is not corrected, this will result in a measurement uncertainty in the processed measurand concentration. Some prior art techniques have been developed to deal with these sources of error. However, the techniques are not well suited for use with TDLs.
One known technique to reduce errors from ambient light is to use interference filters to block a large amount of the broadband light from an ambient light source. However, it is unavoidable that a small but significant amount of ambient light will get through the filter pass band used for the measurement. A prior art technique for negating the error introduced by this residual ambient light is to periodically turn off the laser. This allows the level of residual ambient light to be measured and corrected for. Specifically, the difference in detector signal between the on and off states indicates the amount of incident laser light, thus allowing the residual ambient light to be determined and compensated for. To make use of this technique, however, the laser must be turned off frequently, since ambient light levels will typically vary unpredictably over relatively short timescales, and this technique is not well suited for use with a tunable diode laser. The problem is that repeatedly turning the diode laser on and off causes significant ohmic heating within the diode laser, which is problematic because ohmic heating also affects the wavelength of the output beam of the laser. This dynamic response is referred to as ohmic heating perturbation. Consequently, when this technique is used with TDLs, the laser wavelength scan suffers from large linearity errors, which distorts the measured absorption line shape.
One possible technique to address the error caused by intrinsic fluctuations in the laser output power is to monitor the laser output power while measurements are being made. The laser output power can be monitored at a ‘neutral’ wavelength that is not absorbed by the measurand or other interfering chemical species, and this signal can be compared with a reference value to allow any variations to be corrected for. This results in the measured signal being standardised to a reference value for the amount of incident light. This technique may not be practical in a congested absorption spectrum and could lead to errors.
An alternative method of compensating for variations in the incident light caused by fluctuations in the laser output power is to simultaneously perform second harmonic wavelength modulation spectroscopy and first harmonic modulation spectroscopy. First and second harmonic modulation spectroscopy is known to those skilled in the art and will not be described further here. By taking the ratio of the second harmonic signal to the first harmonic signal, a quantity is obtained that is theoretically independent of the incident light variation for the optically thin regime i.e. weak absorption. However, the inclusion of the first harmonic signal negates the enhanced spectral resolution advantage of using the second harmonic signal alone. Therefore, any interfering background absorption will cause measurement uncertainty of the ratio. This means that this technique is not well suited to situations where many different species are present alongside the measurand, particularly if these species have absorption lines close to or even overlapping those of the measurand. This is often the case in a process gas that is output from an industrial process, for example. In addition, the implementation is more difficult than a standard wavelength modulation spectroscopy system as two separate signal processing channels are required for simultaneous first and second harmonic detection.
A further complication with this technique when used with TDLs is that the residual amplitude modulation (RAM), which is always present when modulating laser diodes using the bias current, adds additional measurement uncertainty to the above ratio. Although the RAM uncertainty can be corrected for by characterising the diode laser intensity modulation behaviour, these modulation characteristics change with the diode laser temperature and bias current settings. The diode laser is normally temperature controlled, which is necessary to achieve frequency stability, and, in an ideal situation, the diode laser temperature is unconditionally stable and hence the modulation characteristics are also stable. However, for a rugged industrial analyser that must cope with wide ambient temperature swings, even the best diode laser temperature control will experience some residual temperature change due to finite thermal control gain. In some cases, the residual temperature change may be significant enough to require retuning of the diode laser frequency onto the measurement absorption line.
Furthermore, the amount of RAM is dependent on the frequency modulation sensitivity of the diode laser as the bias current modulation amplitude is adjusted to achieve sufficient frequency modulation range in order to obtain adequate signal from the wavelength modulation spectroscopy. However, the frequency modulation sensitivity varies among different types of diode lasers. For example, distributed feedback (DFB) structure laser diodes have comparatively low frequency modulation sensitivity, typically less than 0.1 cm−1 per milliamp bias current resulting in large RAM, whereas, vertical cavity semiconductor emitting lasers (VCSEL) tend to have much larger frequency modulation sensitivity, typically greater than 5 cm−1 per milliamp bias current resulting in much lower RAM. There remains a need for an absorption spectroscopy gas analyser system that can produce highly accurate measurements despite fluctuations in the intensity of the transmitted light. There is also a need for such an analyser system that is able to produce highly accurate measurements in a harsh environment as may typically be found in many industrial processes, such as in a furnace or furnace exhaust pipe.